Weird star explodes over and over, cheating death
An exploding star in deep space refuses to die.
This supernova is named iPTF14hls. It has erupted continuously for the last three years. And it may have had two other outbursts in the past.
This tireless supernova could be the first example of a proposed explosion that involves burning particles and their oppositely charged counterparts within the core of a star. Or it could be something else. It could be something astronomers have never seen and don’t yet understand.
Scientists described this oddity in the November 9, 2017 Nature.
“A supernova is supposed to be a one-time thing,” says Iair Arcavi. He is an astrophysicist at the University of California, Santa Barbara. With such objects, the star is just supposed to explode. That’s it. “It’s dead. It’s done. It can’t explode again,” he notes. But not iPTF14hls. It’s “the weirdest supernova we’ve ever seen … It’s like the star that keeps on dying.”
Astronomers discovered it in September 2014 using the Intermediate Palomar Transient Factory. It scans the sky regularly with a telescope at the Palomar Observatory near San Diego. iPTF14hls looked like an ordinary type of supernova in a galaxy about 500 million light-years away. These explosions, called type 2 supernovas, mark the death throes of a star having a mass of between eight and about 50 times that of our sun. Typically these will glow for about 100 days before starting to dim.
The first sign that iPTF14hls was unusual came a few weeks after its discovery. It started growing brighter. Then it dimmed. Then it brightened. The star went through at least five of those irregular cycles of brightening and dimming.
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And, there was something even stranger. Data collected from September 2014 to June 2016 show that the supernova remained bright for more than 600 days, Arcavi and his colleagues now report. Its eruption is only now showing signs of winding down. It may have already been in progress when it was discovered. If so, its explosive swan song may have been going on for even longer.
“That’s just unheard of,” says Stanford Woosley. He is a theoretical physicist at the University of California, Santa Cruz, who was not involved in the discovery. “Ordinary supernovae don’t do that!”
Normally, layers of gas get kicked out of an exploding star. When they do, they slow and cool as they expand. But iPTF14hls is different. It’s toasty — about 5,700° Celsius (10,292° Fahrenheit). This dying star maintained that temperature for the entire time it was observed. And its outer gas layers did not slow down as they should have. That means that this gas may have already cooled and slowed. It probably had been expelled in an earlier, superpowerful eruption. When? That eruption probably took place unseen between 2010 and 2014, the team suggests.
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There’s some history to this star too. Data on photographic plates from the Palomar Observatory showed yet another bright burst in the same part of the sky in 1954. That might have been an earlier flash by this star.
Exploding again and again
How could a star explode over and over?
One theory suggests that stars between 95 and 130 times the mass of the sun can explode several times. To do so, a star must apparently get so hot that a lot of gamma rays — the most energetic form of light — zip about within it. Those gamma rays help to keep the star from collapsing under its own gravity. The star can get so hot, however, that its heat can transform gamma rays into electrons and their oppositely charged counterparts, positrons. Without internal energy from the gamma rays, the star’s core collapses and gets even hotter. That collapse can trigger a partial explosion, in which the star blows off a large share of its mass. But after this explosion, the electrons and positrons might recombine into gamma rays that can hold up the remaining stellar core.
The star can blow up this way several times, the idea goes. Finally, the fiery ball of gas dies in one last supernova. Eventually, its remains would collapse to create a black hole having about 40 times the mass of the sun.
But there’s a snag with this idea for iPTF14hls.
The theory predicts that such a star should blow off all of its hydrogen in the first explosion. iPTF14hls expelled 50 times the mass of the sun in hydrogen in the explosion in 2014. The amount of energy in the most recent explosion is also greater than it should be.
Woosley thinks the star could be a magnetar. These are highly magnetic, rapidly rotating stellar corpses. One could glow continuously for around two years. Still, that wouldn’t explain the 1954 eruption. Woosley says he hopes the most recent data will help determine which theory is right. Or, it could reveal that physicists need to come up with yet another explanation.
Either way, the show may be ending. The latest data show iPTF14hls is finally fading, Arcavi reports. As the outer layers of gas cool and become transparent, they could reveal whatever is at the explosion’s core. The team plans to just keep watching.
“I am not making any more predictions about this thing,” Arcavi says. “It surprised us every time.”
(for more about Power Words, click here)
astronomy The area of science that deals with celestial objects, space and the physical universe. People who work in this field are called astronomers.
astrophysics An area of astronomy that deals with understanding the physical nature of stars and other objects in space. People who work in this field are known as astrophysicists.
black hole A region of space having a gravitational field so intense that no matter or radiation (including light) can escape.
colleague Someone who works with another; a co-worker or team member.
core Something — usually round-shaped — in the center of an object.
corpse The body of a dead human. Also sometimes used to describe the remains of some inanimate object (such as a star).
electron A negatively charged particle, usually found orbiting the outer regions of an atom; also, the carrier of electricity within solids.
galaxy A massive group of stars bound together by gravity. Galaxies, which each typically include between 10 million and 100 trillion stars, also include clouds of gas, dust and the remnants of exploded stars.
gamma rays High-energy radiation often generated by processes in and around exploding stars. Gamma rays are the most energetic form of light.
gravity The force that attracts anything with mass, or bulk, toward any other thing with mass. The more mass that something has, the greater its gravity.
hydrogen The lightest element in the universe. As a gas, it is colorless, odorless and highly flammable. It’s an integral part of many fuels, fats and chemicals that make up living tissues.
light-year The distance light travels in one year, about 9.48 trillion kilometers (almost 6 trillion miles). To get some idea of this length, imagine a rope long enough to wrap around the Earth. It would be a little over 40,000 kilometers (24,900 miles) long. Lay it out straight. Now lay another 236 million more that are the same length, end-to-end, right after the first. The total distance they now span would equal one light-year.
mass A number that shows how much an object resists speeding up and slowing down — basically a measure of how much matter that object is made from.
observatory (in astronomy) The building or structure (such as a satellite) that houses one or more telescopes.
particle A minute amount of something.
physicist A scientist who studies the nature and properties of matter and energy.
positron A subatomic particle with the mass of an electron, but a positive electrical charge. It is the antimatter counterpart to the electron. So when electrons and positrons collide, they annihilate each other, releasing energy.
star The basic building block from which galaxies are made. Stars develop when gravity compacts clouds of gas. When they become dense enough to sustain nuclear-fusion reactions, stars will emit light and sometimes other forms of electromagnetic radiation. The sun is our closest star.
stellar An adjective that means of or relating to stars.
sun The star at the center of Earth’s solar system. It’s an average size star about 26,000 light-years from the center of the Milky Way galaxy. Also a term for any sunlike star.
supernova type 2 (plural: supernovae or supernovas) A condition that occurs when nuclear fusion can no longer continue in the core of a massive star. The reason? The star’s core has burned through its fuel.
swan song A phrase meant to portray the last phase of some event. It comes from a mistaken legend that held swans remain mute until just before they are about to die. Then they were supposed to sing one sad, sweet song. In fact, all swans vocalize throughout their lives, even the so-called "mute" swans.
telescope Usually a light-collecting instrument that makes distant objects appear nearer through the use of lenses or a combination of curved mirrors and lenses. Some, however, collect radio emissions (energy from a different portion of the electromagnetic spectrum) through a network of antennas.
theoretical An adjective for an analysis or assessment of something that based on pre-existing knowledge of how things behave. It is not based on experimental trials. Theoretical research tends to use math — usually performed by computers — to predict how or what will occur for some specified series of conditions. Experimental testing or observations of natural systems will then be needed to confirm what had been predicted.
theory (in science) A description of some aspect of the natural world based on extensive observations, tests and reason. A theory can also be a way of organizing a broad body of knowledge that applies in a broad range of circumstances to explain what will happen. Unlike the common definition of theory, a theory in science is not just a hunch. Ideas or conclusions that are based on a theory — and not yet on firm data or observations — are referred to as theoretical. Scientists who use mathematics and/or existing data to project what might happen in new situations are known as theorists.
transient Lasting or taking place for a relatively short period of time.